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Thread: Diffie Hellman Key exchange implemetation

  1. #1
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    Post Diffie Hellman Key exchange implemetation

    when running the following code it runs one time correctly and after that when I try to run it again, it fails.
    Any Help. I'm using jGRASP editor
    Thanks




    import java.util.*;
    import java.math.*;
     
     
    public class DiffieHellmanBigInt
    {  
     
       public static void main(String args[]) 
       {
     
          int p;
     
          System.out.println("Enter the number of bits to compute the value of the Prime Number:");
          Scanner userInput = new Scanner(System.in);
          int ans = Integer.parseInt(userInput.next());
     
          p = getPrime(ans);
     
          System.out.println("The value of p selected:"+ p);
     
     
           //Random numbers Generation
          Random rand=new Random();
          int g = rand.nextInt((p - 2) + 1) + 2; //between 2 and (p-1)
          System.out.println("The value of g selected: "+ g);
     
          int a = rand.nextInt(99); //generating value of a
          System.out.println("The value of a selected by Alice: "+ a);
     
     
          int b = rand.nextInt(99);//generating value of b
          System.out.println("The value of b selected by Bob: "+ b);
     
           // A's calculation.
          int comput_A=(int)Math.pow(g, a)%p;      
          System.out.println("The value of A sent to Bob by Alice: "+ comput_A);          
     
           // B's calculation.
          int comput_B = (int)Math.pow(g, b)%p;   
          System.out.println("The value of B sent to Alice by Bob: "+ comput_B);
     
     
          int KeyA = (int) Math.pow(comput_B, a)%p;
          int KeyB = (int)Math.pow(comput_A, b)%p;
     
          if(KeyA==KeyB)
          {
          System.out.println("The value of key shared between Alice and Bob: "+ KeyA);
          }
          else
          {
           System.out.println("There is an Error"); 
     
         // System.out.println("The Key A calculates is "+KeyA);
          //System.out.println("The Key B calculates is "+KeyB);
          }
       }
    // evaluate if a number is prime or not  
     
       public static boolean isPrime(int n) 
       {
          for(int i=2;i<n;i++) 
          {
             if(n%i==0)
             {
                return false;
             }
          }
          return true;
       }
     
       // find a prime number
     
       public static int getPrime(int ans) 
       {
          boolean foundPrime= false;
          Random rand_pr = new Random();
     
          if(!foundPrime)
          {
             int num1 = (int)Math.pow(2, ans-1);
             int num2 = (int)Math.pow(2, ans);           
             int  pr = rand_pr.nextInt(num2 - num1 + 1) + num1;
     
             if (isPrime(pr))         
             { 
               return pr; 
             }
          }        
               return 0;        
     
        }     
    }








    Output 1:
    Enter the number of bits to compute the value of the Prime Number:
    5
    The value of p selected:19
    The value of g selected: 3
    The value of a selected by Alice: 57
    The value of b selected by Bob: 88
    The value of A sent to Bob by Alice: 2
    The value of B sent to Alice by Bob: 2
    The value of key shared between Alice and Bob: 2



    Output 2:

    Enter the number of bits to compute the value of the Prime Number:
    5
    The value of p selected:0
    Exception in thread "main" java.lang.IllegalArgumentException: bound must be positive
    at java.util.Random.nextInt(Random.java:388)
    at DiffieHellmanBigInt.main(DiffieHellmanBigInt.java: 24)

    ----jGRASP wedge2: exit code for process is 1.
    Last edited by shahinr2; April 1st, 2018 at 08:12 PM. Reason: added code tags

  2. #2
    Super Moderator Norm's Avatar
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    Default Re: Diffie Hellman Key exchange implemetation

    it fails
    Please explain.
    Post any output that shows what you are talking about.
    If you don't understand my answer, don't ignore it, ask a question.

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